This invention relates to a method for inhibiting polymerization of acrylic-functional silanes. More particularly, this invention relates to a polymerization inhibitor that can inhibit the polymerization of acrylic-functional silanes.
Acrylic-functional silanes, e.g., acryloxy group-containing silanes, methacryloxy group-containing silanes, and the like, readily react with radical-polymerizable monomers, such as methyl methacrylate and styrene, and as a consequence are used as starting materials for copolymers derived from these monomers and as modifiers for polymers obtained from these monomers.
Acrylic-functional silanes can be prepared by an addition reaction between a SiH-functional halosilane and the acrylate or methacrylate ester of an alcohol or phenol that contains an aliphatically unsaturated bond. The desired acrylic-functional silane is then isolated from the resulting mixture by distillative purification. Refer in this regard to, for example, Japanese Patent Application Kokai Number Hei 5-301881 (301,881/1993). However, this addition reaction and isolation by distillative purification has been encumbered by severe difficulties because compounds of this type readily polymerize upon heating and as a result can become highly polymerized during the reaction and distillation stages. This problem has made it quite difficult to obtain high-purity acrylic-functional silanes in high yields. Therefore it is necessary to run the addition reaction in these methods at a reaction temperature at which thermal polymerization will not occur. However, this type of temperature control is quite difficult and the reaction product often ends up becoming highly polymerized and gelled.
The addition of a hindered phenol, amine compound, or quinone compound to the reaction system has also been proposed as a method for inhibiting the polymerization and gelation of acrylic-functional silanes. For example, Japanese Patent Application Kokai number Hei 7-25907 (25,907/1995) describes a substantial improvement in thermal stability for the addition of 2,6-di-tert-butyl-4-hydroxymethylphenol to a crude mixture prepared from .gamma.-methacryloxypropyltrichlorosilane and methanol whose main component is .gamma.-methacryloxypropyltrimethoxysilane. This inhibitor, however, does not provide a complete inhibition of gelation during the addition reaction between a SiH-functional halosilane and the acrylate or methacrylate ester of an aliphatically unsaturated alcohol or phenol. In addition, Japanese Patent Application Kokai Number Hei 5-186478 (186,478/1993) has proposed the use of N,N-dialkylaminomethylenephenol as a polymerization inhibitor. This inhibitor performs relatively well in terms of inhibiting the polymerization of acrylic-functional silanes and halosilanes. However, it acts as a catalyst poison for the platinum catalysts used as the addition-reaction catalyst in the preparation of these silanes by the above-described addition reaction, with the result that large amounts of platinum catalyst must be employed in order to complete the reaction.
The inventors achieved the present invention as a result of extensive investigations directed to solving the problems described above. Specifically, the object of the present invention is to provide a novel polymerization inhibitor that can inhibit the polymerization of acrylic-functional silane during their synthesis, purification, and storage. The polymerization inhibitors according to the present invention has an inhibiting effect superior to that of known polymerization inhibitors.